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Abstract
Benefiting from unique electronic and optoelectronic properties, tin selenide nanostructures show great potential for applications in energy storage and photovoltaic devices. A novel nanostructured solid-state optoelectronic device based on inorganic/organic semiconductor heterojunction of the arrays of TiO2/SnSe2 core–shell nanocable structure is constructed. To the best of our knowledge, TiO2/SnSe2 nanocables were synthesized by the ion replacement method for the first time. The structural features and morphology of the nanocables were characterized through Raman spectroscopy and scanning electron microscope (SEM). The absorption property had a significant increase with SnSe2 loading at TiO2 nanorods, indicating that the SnSe2 served as the active absorption layer and TiO2 offered a pathway for electron transport. This work offers a novel, practical and promising method for achieving SnSe2-based devices in the application of optic-electronics.
© 2017 Optical Society of America
1. Introduction
As a new 2D semiconducting material, SnSe2, with similar configuration as MoS2, has brilliant application prospects as the anode material for infrared optoelectronic devices, memory switching devices, lithium ion batteries and solar cells [1–8]. Different from the intensively studied transition metal dichalcogenides (TMDs), SnSe2 belongs to the family of IVA-VIA group, which is earth-abundant and environment friendly, and thus has an obvious future advantage in nanoelectronics and optoelectronics because of its low cost and low toxicity [9–13]. Similar to other IV–VI binary semiconductor nanocrystals (NCs), SnSe2 crystallizes in a hexagonal lattice with a van der Waals gap separating the (001) planes, in which the Sn planes are sandwiched between the bilayer Se planes [14]. In addition, the band gaps of SnSe2 show layer-sensitive features, that the indirect fundamental band gaps of few-layers and bulk are ranging from 1.07 eV to 1.69 eV, whilst the direct ones are from 1.84 eV to 2.04 eV [15]. Previous report about mechanical exfoliated SnSe2 crystals demonstrated that it has a high carrier mobility of 8.6 cm2 V−1 s−1 in atmospheric environment [16]. However, there are few studies on photochemical batteries for SnSe2. TiO2 nanostructures combined with other materials have been widely used in optoelectronic devices, including SnSe2 quantum dot sensitized TiO2 solar cells, TiO2-SiO2 photocatalysis, etc [17–20,31]. Nanoparticles of optoelectronic materials can produce quantum local effects [17]. In the other words, these nanoparticles will further improve the photoelectric conversion efficiency, suggesting that the TiO2/SnSe2 core-shell structure should have a high optoelectronic performance [21]. To the best of our knowledge, there is few researche of quantum dot sensitized solar cell based on TiO2-SnSe2 nanorod arrays. Since SnSe2 dissolving polysulfide electrolyte might be a reason for restricting its photochemical battery research, and consequently, we designed a solid-state quantum dot solar cell based on TiO2/SnSe2 core-shell nanocable/spiro-MeOTAD.
In this work, the solid-state optoelectronic device based structure of TiO2/SnSe2 core–shell nanocable/spiro-MeOTAD was synthesized. SnSe2 shells were prepared by sample ion replacement method. SnSe2 and spiro-MeOTAD were the photo active layer and the hole conduction layer, respectively. The morphology and structure were measured by scanning electron microscope (SEM) and transmission electron microscope (TEM). There have been a significant decrement of the transmittance when SnSe2 loaded at TiO2 nanorod. Therefore, photoelectric response of J-V curves have been obtained.
2. Experimental
TiO2/SnSe2 core-shell nanocables are composed with SnSe2 shells and TiO2 nanorods, in which the arrays of nanorodhave been grown directly on a fluorinedoped tin oxide (FTO)-coated glass using the hydrothermal methods [22, 23]: The FTO substrates were ultrasonically cleaned for 10 min in a mixed solution consist of deionized water, acetone, and 2-propanol with volume ratios of 1:1:1. 50 mL of deionized water was mixed with 40 mL of concentrated hydrochloric acid (mass fraction is 36-38%). After stirring at ambient temperature for 5 min, 400 μL of titanium tetrachloride was added to the mixture. The feedstock, prepared as previously described, was injected into a stainless steel autoclave with a Teflon lining. The FTO substrates were placed at an angle against the Teflon liner wall with the conducting side facing down. The hydrothermal synthesis was performed by heating the autoclave with an oven at 180 °C for 2 h. After synthesis, the autoclave was cooled down to room temperature under flowing water, and then, the FTO substrates were washed extensively with deionized water, and dried in open air.
Based on the facile hydrothermal method, TiO2 nanorod arrays have been successfully grown on fluorinedoped tin oxide glass (as depicted in Fig. 1(a)). Arrays of TiO2/ZnO core–shell nanocables were prepared by spin coating a solution of zinc acetate ethanol (5.0 mM) in isopropyl alcohol on the substrate with TiO2 nanorod arrays and subsequently were annealed at 500 °C for 1 h. Afterwards, arrays of TiO2/ZnSe core–shell nanocables were prepared by immersing the ZnO nanoparticals in a Se2- ionsolution (6.0 mM) prepared by reacting Se powder with NaBH4 at 55 °C for 2.5 h. As structural sketch map displayed in Fig. 1(b), arrays of TiO2/SnSe2 core–shell nanocables were prepared by further reacting the TiO2/ZnSe core–shell nanocable arrays with a SnCl2 and sodium citrate mixture solution (25.0 mM) for 12 h at 90 °C [24, 25].
Fig. 1 Schematic of (a) bare TiO2 nanorod arrays on FTO and (b) SnSe2-TiO2 nanostructure on FTO. Schematic device structures of (c) the TiO2 nanorod arrays/spiro-MeOTAD inorganic/organic heterojunction and (d) the arrays of TiO2/SnSe2 core–shell nanocables /spiro-MeOTAD inorganic/organic heterojunction.
A spiro-MeOTAD (Merck KGaA)/chlorobenzene (180 mg/1 ml) solution was mixed with 37.5 ml Li-bis (tri-uoromethanesulfonyl) imide (Li-TFSI)/acetonitrile (170 mg/1 ml) and 17.5 ml 4-tertiary butyl phenol (4-TBP). A gold counter electrode was deposited on the sample by thermal evaporation [26]. In this work, we constructed two kinds of optoelectronic devices, which used the TiO2 nanorod arrays and the arrays of TiO2/SnSe2 core–shell nanocables as the photoanodes, respectively.
3. Discussion
The microstructure of the sample was further analyzed by using SEM and TEM. Figure 2(a) shows the typical SEM images of TiO2 nanorod arrays on a FTO glass substrate. Figure 2(b) shows the magnified part of the SnSe2 shells grown on TiO2 nanorod arrays. The core/shell structure can be observed clearly in Fig. 2(c). Figure 2(d) can give further insight into the details of the structure. Observed in the figures, the surfaces of the TiO2 nanorod arrays are covered fully and completely with SnSe2 nanoparticles.
Fig. 2 SEM images of TiO2 nanorod arrays (a) grown on the FTO substrates,(b) a magnified part of the TiO2/SnSe2 core-shell nanocables on the FTO glass substrate. TEM images of the TiO2/SnSe2 core-shell nanocable (c) showing a cabled structure. (d) High-resolution TEM image showing the crystal and amorphous nature of the core and the sheath, respectively.
Figure 3(a) shows the schematic diagram of the Raman spectra measurement of the generated structures. The Raman spectra of the arrays of TiO2/SnSe2 core–shell nanocables were excited by a 532-nm laser. Raman peaks of SnSe2 and TiO2 were observed clearly as shown in Fig. 3(d) [15, 27]. The Eg mode of SnSe2, is doubly degenerate and has been characterized by an in-plane stretching behavior. The A1g mode of SnSe2 has been characterized by an out of plane stretching of the selenium atoms. Diagram for the in- and out-plane Raman models of SnSe2 was shown in Fig. 3(b), where the gray symbols are Sn and the red ones are Se, of which the vibration modes induced two main Raman modes [28, 29]. The Eg mode of TiO2 is doubly degenerated and has been characterized by an in-plane stretching behavior. The A1g mode of TiO2 has been led by an out of plane stretching of the titanium atoms. In- and out-plane Raman models of TiO2 were shown in Fig. 3(c), in which the silver symbols are Ti and the blue ones are O, inducing two main Raman modes [27]. Figure 2(d) shows the measured Raman spectrum of the arrays of TiO2/SnSe2 core-shell nanocables. The Eg and A1g modes of SnSe2 (red letters) have been observed with the positions of 106.2 cm−1 and 180.2cm−1. And the Eg and A1g modes of TiO2 (blue letters) have been observed with the positions of 433 cm−1 and 607.2 cm−1, respectively, indicating that the sample is composed by SnSe2 and TiO2 distinctly. There is a peak between the A1g mode of SnSe2 and Eg mode of TiO2, which have been observed with the position of 252.6 cm−1, could be the B1g mode of TiO2 [30].
Fig. 3 The schematic diagram of the Raman spectra measurement (a); The schematic of in-and out-plane Raman modes of SnSe2 (b) and TiO2 (c); room-temperature Raman spectra of the arrays of TiO2/SnSe2 core–shell nanocables (d).
The light transmission properties of the arrays of TiO2/SnSe2 core–shell nanocables and the TiO2 nanorod arrays were investigated by UV-visible transmittance spectra over a wavelength range from 300 to 800 nm. Figure 4(a) shows the optical transmittance spectra of the sample. A sharp decrease can be observed on the blue line at approximately 420 nm, which is the exhibition of transmittance curve of TiO2 nanorod arrays. Compared with the TiO2/SnSe2 core–shell nanocables (red curve), the transmittance increased slower than TiO2 indicating that a calculated enhanced absorption has been caused by the induced SnSe2 nanoparticles. Figure 4(b) shows the (αhν)1/2-hv spectra of the arrays of the TiO2/SnSe2 core–shell nanocables and (αhν)2-hv spectra of TiO2. Draw a tangent line in the linear region of the spectra. The line is extrapolated to hν-axis; the intercept of 1.4 eV was obtained for SnSe2, the intercept of 3.15 eV was obtained for TiO2. We can probably get optical band gap of TiO2 and SnSe2, which is 3.15 eV and 1.4 eV.
Fig. 4 (a) UV-vis transmittance spectra and (b) (αhv)2-hv spectra of the arrays of the TiO2/SnSe2 core–shell nanocables and the TiO2 nanorod arrays.
In addition, using the arrays of TiO2 nanorods and TiO2/SnSe2 core–shell nanocables as the photoanodes, we fabricated two kinds of optoelectronic devices to evaluate their photovoltaic performances. Figures 5(a) and 5(b) show the current density–voltage (J-V) characteristics of the optoelectronic devices based on TiO2 nanorod arrays and TiO2/SnSe2 core–shell nanocables, respectively. Compared with the values of short-circuit current (the intersections of curves and y-axes) and open-circuit voltage (the intersections of curves and x-axes), the device based on TiO2/SnSe2 core–shell nanocables owns a superior performance batter than TiO2 one. This phenomenon is due to the visible light is utilized by SnSe2 instead of TiO2. The result of J-V curves are similar to the previous work based on the material of SnSe2 [17].
Fig. 5 Density–voltage (J–V) characteristics of the optoelectronic device of TiO2 nanorod arrays (a) and TiO2/SnSe2 core–shell nanocables (b); (c)Schematic energy band diagram and the electron-transfer processes of the arrays of TiO2/SnSe2 core–shell nanocables /spiro-MeOTAD heterojunction.
The schematic energy band diagram of the device (i.e. FTO/TiO2/SnSe2/spiro-MeOTAD/Au), is presented in Fig. 5(c). When the device was illuminated, electrons in SnSe2 were excited from the valence band energy levels to the conduction, and electron-hole pairs were generated in the SnSe2. A built-in potential, induced by the difference between conduction bands of SnSe and TiO2, separated the electron-hole pairs. Meanwhile, the separated electrons transferred from the conduction band of SnSe2 into the TiO2 nanrods. They were collected by the FTO, because work function of FTO matched with the conduction band of TiO2 [32]. These electrons moved into the external circuit and then came back to the Au layer from the detector. The holes moved from the valence band of SnSe2 into highest occupied molecular orbital (HOMO) energy levels of spiro-MeOTAD at the interface of the SnSe2/spiro-MeOTAD, subsequently moved into Au layer at the interface of the spiro-MeOTAD/Au.
4. Conclusion
In summary, arrays of TiO2/SnSe2 core–shell nanocables on FTO glass substrates have been achieved using a simple hydrothermal and facile ion-exchange approach, which endows the nanocable structure with complete coverage and effective loading of the SnSe2 nanoparticles with enhanced light absorption. Based on the arrays of TiO2/SnSe2 core–shell nanocables, a nano-structured solid-state optoelectronic device consisted of a semiconductor heterojunction of TiO2/SnSe2 core–shell nanocable/spiro-MeOTAD is constructed. The excellent performances of the device pave a new avenue for the SnSe2-based devices in the application of optic-electronics.
Funding
National Natural Science Foundation of China (No. 11574185); Shandong Province Science and Technology Development Plan (No. 2012GGB14025).
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